Abstract:

Strata of the Triassic Induan in the Upper-Yangtze region, i.e. the littoral-facies sandstones and mudstones of the Feixianguan Formation in the western part and the ramp carbonates of the Daye Formation in the eastern part, represent a developing process of a ramp carbonate platform. Controlled by the two transgressive-regressive processes in the Induan age of the Early Triassic, the oolitic-bank limestones within the Daye Formation in the Upper-Yangtze region have been undergone the obvious progradation from the west to the east. Resulting from this sedimentary process, a set of oolitic-bank limestones with the thickness of more than 40 meters is developed in the top part of the Triassic Daye Formation at the Lichuan section. For this set of oolitic-bank limestones, the larger grain size and the diversity of morphology make it become a useful example to understand the origin of oolites. Further, the diverse configurations including the circular, the sub-circular, the irregular and the composite shapes of oolites in the top part of the Triassic Daye Formation at the Lichuan section, especially for the development of the giant oolites that are unusual in the Phanerozoic, provide an important clue for the understanding of both the sedimentary response to the mass extinction at the turn from the Permian to the Triassic and the evolving carbonate world that is reflected by the oolite whose origin remains uncertain.

Key words: giant oolite, oolitic diversity, Daye Formation, Triassic, Lichuan section in Hubei Province